In order to improve power density, a majority of diesel engines have intake manifold pressures above atmospheric conditions. This allows for the introduction of more fuel, which results in more power. It is common for diesel engines to receive charged air from a turbocharger (e.g., a Fixed Geometry Turbocharger “FGT”, a Variable Geometry Turbocharger “VGT” or a combination system including both). A turbocharger develops boost by converting exhaust gas energy into power, which is used to compress the intake charge. The primary advantage of using a turbocharger is its ability to harness waste heat from the engine. This advantage brings with it the disadvantage of increased backpressure on the engine and lower quality heat for exhaust aftertreatment devices. A turbocharged engine may also have a significant performance disadvantage as well. Since turbochargers rely on exhaust gas energy, turbochargers tend to struggle to produce adequate levels of boost at lower engine speeds. A transient load step, especially when starting from a relatively low engine speed, can result in a delayed response from the engine, a phenomenon called “lag”. With a turbocharger, the exhaust enthalpy content can be too low at low load conditions to drive the turbocharger and under these conditions, when the accelerator pedal is depressed, the vehicle may respond slowly due to the turbocharger “lag” as the turbocharger accelerates up to speed. The industry has tried different approaches to adapt to the turbocharger lag experiences during a transient acceleration event. One common approach to improve transient performance at low engine speeds has been the use of a smaller turbocharger turbine with less inertia. The improved transient performance, however, normally comes at the expense of poorer fuel efficiency at high speeds since the smaller turbine can cause higher exhaust restriction. Another solution that has been proposed that may provide similar engine performance is supercharging.
Superchargers, in contrast to turbochargers, use energy from the engine crankshaft. Unlike a turbocharger, which relies on exhaust gas energy to create boost, a supercharger is directly coupled to the engine. Therefore, the supercharger speed increases proportionally with engine speed. This greatly reduces the lag that may be present in a turbocharged engine, dramatically improving low speed engine response. Thus, superchargers can provide air-on-demand for effective air-fuel ratio control. The ability to maintain high air-fuel ratios during transient loads leads to superior transient performance since more fuel can be injected to produce more power with reduced smog production. For example, a supercharger such as the TVS® system manufactured by Eaton Corporation is a positive displacement pump that has shown significantly improved efficiencies over prior models. However, no matter how efficient, the advantages from a supercharger typically come at the cost of slightly higher fuel consumption versus a comparably-sized turbocharger. The higher fuel consumption is normally attributable to mechanical losses characteristic of a supercharger and its inability to recover waste heat energy.
The light, medium, and heavy-duty engine markets have both stringent regulatory targets and customer demand for improved fuel efficiency. Two approaches that may be used to meet fuel efficiency targets are engine downspeeding and downsizing.
What is needed is an engine boosting system that combines the benefits of a supercharger with those of a turbocharger, to meet the fuel efficiency targets, while maintaining or improving vehicle performance over a baseline single charging system or a twin turbo system.